Method of manufacturing a nuclear fuel pellet by recycling an irradiated oxide fuel pellet

ABSTRACT

A method of manufacturing new UO 2 -based fuel pellets by recycling irradiated UO 2 -based fuel pellets. Irradiated UO 2 -based fuel pellets are oxidized so as to make U 3 O 8 -based powder, and then the U 3 O 8  -based powder is mixed with an additive which contains at least one oxide of an element selected from the group consisting of niobium, titanium, vanadium, aluminum, magnesium, chromium, silicon and lithium. Green pellets are formed from the mixed powder, and then sintered, preferably at 1500° C. or higher, in a reducing gas atmosphere to produce UO 2 -based fuel pellets with high densities.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of manufacturing UO₂-based fuelpellets by recycling irradiated UO₂-based fuel pellets. Moreparticularly this invention relates to a method of making U₃O₈-basedpowder from irradiated UO₂-based fuel pellets, mixing the U₃O₈-basedpowder with an additive, pressing and sintering the product to producethe UO₂-based fuel pellets.

2. Definitions of Terminology

A fresh fuel pellet consists of uranium dioxide (UO₂) or a mixed oxideof uranium dioxide (UO₂) and plutonium dioxide (PuO₂), which mixed oxideis hereinafter referred to as “(U, Pu)O₂”. In this application, the term“UO₂-based” refers to both UO₂- or (U, Pu)O₂-containing products, andthe term “U₃O₈-based” refers to both U₃O₈- or (U, Pu)₃O₈-containingproducts.

3. Description of Related Art

As a UO₂-based fuel pellet is irradiated in a nuclear reactor, thefissile material in the fuel pellet is depleted and fission products areproduced. An irradiated UO₂-based fuel pellet therefore comprisesfissile materials and fission products, of which concentrations aremainly dependent on design burnup specifications and the initial amountof fissile materials. An irradiated fuel pellet discharged from a lightwater reactor normally has fissile materials of higher than 1% by weightof the irradiated fuel pellet, so it is worthwhile to recycled the fuelpellet for reuse.

According to the literature (G. E. Brand and E. W. Murbach, NAA-SR-11389(1965)), a UO₂-based fuel pellet which has been irradiated in a lightwater reactor can be treated in a so-called AIROX cycle, to refabricatethe UO₂-based fuel pellet. The AIROX cycle comprises the steps ofoxidizing irradiated UO₂-based fuel pellets to U₃O₈-based powder, makingsinterable UO₂-based powder from the U₃O₈-based powder, mixing thesinterable UO₂-based powder with enriched fresh UO₂ powder, makinggranules of the mixed powder, pressing the granules into green pellets,and sintering the green pellets to fabricate UO₂-based fuel pellets forreuse in a light water reactor.

The art discloses several methods of making sinterable UO₂-based powderfrom irradiated UO₂-based fuel pellets for use in the AIROX cycle. U.S.Pat. No. 3,140,151, discloses a method of making a sinterable UO₂-basedpowder comprising oxidizing the irradiated UO₂-based fuel pellet toU₃O₈-based powder in air at a temperature in the range of 300° C. to500° C., reducing the U₃O₈-based powder to UO₂-based powder at atemperature in the range of 500° C. to 800° C., and repeating theoxidation and reduction steps 3 to 5 times to produce the sinterableUO₂-based powder. This UO₂-based powder was able to be sintered toproduce UO₂-based fuel pellets.

A green pellet consisting of U₃O₈-based powder which is produced fromirradiated UO₂-based pellets through one round of oxidation can besintered only up to about 80% theoretical density (TD), and a fuelpellet having such a low density cannot be used in a nuclear reactor,since normal fuel design specifications require the pellet density to beat least about 94% TD. Therefore, oxidation and reduction of theU₃O₈-based powder needed to be conducted for many more rounds to enhanceits sinterability. In addition, during the oxidation of UO₂-basedpellets or powder to U₃O₈-based powder, the irradiated UO₂-based pelletsor powder must be readily pulverized or comminuted to finer powder,since the phase transition of cubic UO₂ to orthorhombic U₃O₈ causes avolume expansion of about 30%, and thus large stress is generated.

A disadvantage of the prior art is that the oxidation and reduction ofUO₂-based powder needs much time and is hard to control. For example,the oxidation rate of UO₂-based powder is vety fast, and thus the heatproduced from the resultant reaction can increase the temperature of thepowder to high temperatures. The UO₂-based powder so produced is poorlysinterable.

SUMMARY OF THE INVENTION

An object of this invention is to provide a method of manufacturingUO₂-based fuel pellets by recycling irradiated UO₂-based fuel pellets.

With the foregoing object and other objects in view, there is providedin accordance with this invention a method of preparing UO₂-based fuelpellets, comprising oxidizing irradiated UO₂-based pellets to makeU₃O₈-based powder; mixing the U₃O₈-based powder with an additivecomprising at least one oxide of an element selected from the groupconsisting of Nb, Ti, V, Al, Mg, Cr, Si and Li; making granules of themixture; pressing the granules into green pellets; and sintering thegreen pellets in a reducing gas atmosphere to produce new UO₂-based fuelpellets.

The method according to the invention is preferably conducted such thatthe additive is in the range of about 0.02% to about 5% by weight of themixture. It is also preferable that the green pellets are sintered at atemperature above 1500° C.

In another method according to the invention, fresh UO₂-based powder isadded to the U₃O₈-based powder to bring the amount of fissile materialswithin design specifications.

An advantage of the present invention is that the U₃O₈-based powderproduced through a single oxidation treatment of irradiated UO₂-basedpellets may be pressed without any further oxidation treatments beingnecessary, and sintered to make new UO₂-based fuel pellets. Accordingly,the 3-5 oxidation and reduction cycles which were necessarily requiredin the prior art are not needed in the present invention. Therefore,fuel manufacturing steps and related costs are much reduced by thepresent invention.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows the manufacturing steps in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

An irradiated UO₂-based fuel pellet is first separated mechanically orphysically from a cladding tube. Thereafter, the irradiated fuel pelletis processed through the manufacturing steps shown in FIG. 1.

The irradiated UO₂-based fuel pellet is oxidized in an oxidizing gas soas to make U₃O₈-based powder containing fission products. Then, theU₃O₈-based powder is mixed with an additive to form a mixture. Themixture is granulated into granules, which are then pressed into a greenpellet. The green pellet is sintered in a reducing gas atmosphere,preferably at a temperature of at least 1500° C. for at least 1 hour. Itis noted that irradiated (U, Pu)O₂ fuel pellets can be processed in thesame way as irradiated UO₂ fuel pellets, since they each have the samecrystal structure.

The detailed description of the method of manufacturing new UO2-basedfuel pellets from irradiated UO₂-based fuel pellets is as follows.

The irradiated UO₂-based fuel pellet is heated in a furnace, preferablyat a temperature in the range of about 300° C. to about 700° C., in anoxidizing gas. The oxidizing gas is preferably selected from the groupconsisting of air; oxygen; a mixture of air and inert gas; and a mixtureof oxygen and inert gas. It has been found that the particle size of theU₃O₈-based powder increases with the oxidation temperature, so it ispreferred to oxidize irradiated UO₂-based fuel pellets in air at atemperature in the range of about 350° C. to about 600° C. Theirradiated UO₂-based fuel pellet is spontaneously pulverized toU₃O₈-based powder by the above treatment, since the lattice volumeexpands by about 30% and a large amount of stress is resultantlygenerated. The U₃O₈-based powder is preferably screened to remove largeagglomerates or pellet fragments, and the U₃O₈-based powder so producedpreferably has an average particle size of about 8 μm.

An irradiated UO₂-based fuel pellet has both gaseous fission productsand solid fission products. Gaseous fission products include xenon andkrypton, and solid fission products include Pu, Ce, Mo, Zr, Nd, Ba andLa. While the gaseous fission products are almost completely removedfrom the irradiated fuel pellet during the above oxidation andpulverization steps, the resulting U₃O₈-based powder still containsamounts of the above-noted solid fission products. If the content offissile materials in the U₃O₈-based powder is less than that required bythe design specification of the fuel pellet to be manufactured, freshUO₂-based powder, enriched or depleted, may be added to the U₃O₈-basedpowder in order to meet the amount of fissile materials required by thedesign specification. In this regard, it is useful to note that freshUO₂ powder, fresh PuO2 powder or a mixed powder of UO₂ and PuO2 (i.e.,(U, Pu)O₂) may be added for the same purpose. The addition of freshUO₂-based powder to the U₃O₈-based powder enhances the density of theUO₂-based fuel pellet which will be manufactured, so there is norestriction on the amount of fresh UO₂ powder which may be added, fromthe viewpoint of pellet manufacturing.

In accordance with the present method, the U₃O₈-based powder is mixedwith an additive comprising at least one oxide of an element selectedfrom the group consisting of Nb, Ti, V, Al, Mg, Cr, Si and Li. Thequantity of the additive is preferably in the range of about 0.02% toabout 5% by weight of the mixture of the U₃O₈-based powder and theadditive.

The mixture is thereafter preferably pre-pressed under about 1 ton/cm²of pressure into slugs, which are broken up into granules having goodflowability. The granules are pressed in a mold, preferably under apressure of higher than about 2 ton/cm², to produce a green pellet ofabout 40% to 70% TD. Before the pre-pressing or the pressing, alubricant such as zinc stearate may optionally be added to the mixtureto decrease the friction between particles during the pressing. A bindermay also be added, if necessary, to increase the strength of the greenpellet. If the mixture of the U₃O₈-based powder and the additive isflowable enough to be directly pressed, the mixture may be pressed intoa green pellet without being granulated.

If lubricants or binders were added to the green pellet, the greenpellet may, at this point in the process, be heated to a temperature ofabout 700° C. for a sufficient time to remove the lubricants or binders.If no lubricants or binders were added, this step is not necessary.

The green pellet is subsequently heated in a reducing gas atmosphere. Ina first variation of the method, heating preferably is conducted at atemperature above about 1500° C. for at least 1 hour. The reducing gasatmosphere is needed to make a stoichiometric UO₂-based fuel pellet, sothe reducing gas preferably comprises a gas selected from the groupconsisting of hydrogen, a mixed gas of hydrogen and at least one inertgas (such as argon and nitrogen), a mixed gas of hydrogen and steam, anda mixed gas of hydrogen and carbon dioxide.

Without the addition of additives in accordance with the presentinvention, a green pellet comprising the U₃O₈-based powder is sinteredto a UO₂-based pellet having a density of only about 80% TD, due to thevery low sinterability of the U₃O₈-based powder. A UO₂-based pellet withsuch a low density cannot be used in a nuclear reactor, because fuelspecifications require the pellet density to be at least about 94% TD.With the addition of the additives in accordance with the presentinvention, a pellet density of at least about 94% TD may be achieved.

Another problem in achieving higher pellet density is the formation ofmicro-cracks in the green pellet during heating in the reducing gas. Thereduction of orthorhombic U₃O₈-based compounds to cubic UO₂-basedcompounds results in a contraction of volume, which in turn causesmicro-cracks to be formed in the green pellet since the volumecontraction is not accommodated at relatively low temperatures. When thereduction of U₃O₈-based compounds to UO₂-based compounds is performed ata high temperature such that the volume contraction can be accommodatedwithout cracking, the green pellet can be sintered to a higher density.Therefore, in a second variation of the method in accordance with theinvention, before the sintering step the green pellet is first heated ina non-reducing gas (preferably to a temperature in the range of about700° C. to 1100° C.), wherein the reduction of U₃O₈-based compounds toUO₂-based compounds is avoided, and then is sintered by heating tohigher temperatures in a reducing gas (preferably to a temperature aboveabout 1500° C. for at least 1 hour). UO₂-based pellets prepared by thisvariation on the present method have an even higher density than theUO₂-based pellet produced only in a reducing gas throughout thesintering step. Preferably, the non-reducing gas comprises at least onegas selected from the group consisting of an inert gas, nitrogen, carbondioxide, air and steam.

A UO₂-based fuel pellet produced in accordance with the presentinvention has a density in the range of about 94% TD to about 98% TD. AUO₂-based fuel pellet which has a density between 94% TD and 96.5% TD issuitable for a light water reactor, and a UO₂-based fuel pellet whichhas a density between 96% TD and 98% TD is suitable for a heavy waterreactor. Thus, the method provided by the present invention is able tofabricate fuel pellets suitable for both types of reactors.

The following examples illustrate preferred methods of fabricatingUO₂-based fuel pellets in accordance with the present invention.However, these examples should be understood to in no way limit thescope of the invention, which is only defined by the appended claims.

EXAMPLE 1

A UO₂ fuel pellet irradiated to 35,000 MWD/MTU in a light water reactorhas compositions of fissile materials and fission products. Theircompositions are calculated with the ORIGEN computer code, and 12 majorelements were selected from all the elements contained in the irradiatedUO₂ fuel pellet. A simulated UO₂ fuel pellet having the same compositionas the irradiated UO₂ fuel pellet was prepared using fresh UO₂ powderand the 12 non-radioactive fission products mentioned above. Fresh UO₂powder was mixed with the pre-determined amounts of the 12 elements, andthe composition of the mixed powder is shown in Table I.

TABLE I OXIDES % BY WEIGHT SrO 9.147 × 10⁻² Y₂O₃ 5.488 × 10⁻² ZrO₂ 4.487× 10⁻¹ MoO₃ 4.737 × 10⁻¹ RuO₂ 3.678 × 10⁻¹ Rh₂O₃ 4.814 × 10⁻² PdO 1.464× 10⁻¹ TeO₂ 5.585 × 10⁻² BaCO₃ 2.552 × 10⁻¹ La₂O₃ 1.926 × 10⁻¹ CeO₂9.186 × 10⁻¹ Nd₂O₃ 6.605 × 10⁻¹ UO₂ 96.286

The mixed powder was bail-milled, pressed and sintered to make simulatedUO₂ fuel pellets. The simulated UO₂ fuel pellets have a density of about96% TD and have the same composition as the irradiated UO₂ fuel pellet,so it can be used as a substitute for the irradiated UO₂ fuel pellet.The simulated UO₂ fuel pellet does not emit radioactive rays, so itcould be treated in an unshielded lab.

The simulated UO₂ fuel pellet was oxidized in flowing air at 400° C. for3 hours to make U₃O₈-based powder, which was then passed through a sievehaving 425 μm openings to remove large agglomerates. The U₃O₈-basedpowder had an average particle size of 8 μm.

The U₃O₈-based powder was mixed uniformly with niobium oxide (Nb_(2O) ₅)as an additive, which quantity was 0.5% by weight of the U₃O₈-basedpowder. In parallel, the U₃O₈-based powder was mixed uniformly withtitanium oxide (TiO₂) as an additive, which quantity was 0.2% by weightof the U₃O₈-based powder. The mixed powders were pre-pressed under apressure of 98 MPa into slugs, which were then broken up into granulesof 425 μm or smaller.

The granules were mixed with zinc stearate of 0.2% by weight of thegranules for lubrication and were pressed into green pellets in a moldunder pressures of 392 MPa, 490 MPa and 588 MPa. Green pellets wereheated to 1700° C. in reducing gas atmospheres for 4 hours and thencooled-down to fabricate UO₂-based fuel pellets. During the heatingprocess, the green pellet was sintered and simultaneously reduced fromU₃O₈ to UO₂. The reducing gas was hydrogen for the green pelletcontaining TiO₂ and was a mixed gas of hydrogen and carbon dioxide forthe green pellet containing Nb₂O₅.

Table II shows the densities of the UO₂-based fuel pellets fabricated inaccordance with the above procedures. In order to show clearly theeffect of the additives, a U₃O₈-based powder containing no additives wasprocessed following the same procedure, and the density is also shownfor comparison in Table II.

TABLE 11 additives pressing pressure green density reducing gas sintereddensity sintered density (wt %) (MPa) (g/cm³) atmosph (vol %) (g/cm³) (%TD) 0.5% Nb₂O₅ 392 5.56 H₂ + 1% CO₂ 10.119 94.39 0.5% Nb₂O₅ 490 5.72H₂ + 1% CO₂ 10.192 95.07 0.5% Nb₂O₅ 588 5.86 H₂ + 1% CO₂ 10.261 95.720.5% Nb₂O₅ 588 5.88 H₂ + 2% CO₂ 10.369 96.72 0.5% Nb₂O₅ 588 5.87 H₂ + 3%CO₂ 10.314 96.21 0.2% TiO₂ 588 5.96 H₂ 10.561 98.52 not added* 588 5.82H₂ 8.0 74.6 *comparative example

EXAMPLE 2

U₃O₈-based powder was prepared in the same way as in Example 1. TheU₃O₈-based powder was mixed with niobium oxide (Nb₂O₅), which contentswere 0.3% and 0.5% by weight of the U₃O₈-based powder, respectively. Inparallel, the U₃O₈-based powder was mixed with titanium oxide (TiO₂),which contents were 0.1% and 0.2% by weight of the U₃O₈-based powder,respectively. The mixed powders were pre-pressed under 98 MPa intoslugs, which were then broken up into granules of 425 μm or smaller.

The granules were mixed with zinc stearate at 0.2% by weight of thegranules for lubrication and were pressed into green pellets in a moldunder 588 MPa. The green pellets were heated to 800° C., 900° C. and1000° C. in argon gas, respectively, for 1 hour, and subsequently heatedin reducing gas atmospheres to 1700° C. for 4 hours. The reducing gaswas hydrogen for the green pellet containing TiO₂ and was a mixed gas ofhydrogen and carbon dioxide for the green pellet containing Nb₂O₅.

Table III shows the densities of the UO₂-based pellets fabricated inaccordance with the above procedures. In order to show clearly theeffect of the additives, a U₃O₈-based powder containing no additives wasprocessed following the same procedure, and the density is also shownfor comparison in Table III.

TABLE III annealing of green pellets additives pressing pressure duringheating reducing gas sintered density sintered density (wt %) (MPa)(temp/gas/time) atmosh (vol %) (g/cm³) (% TD) 0.3% Nb₂O₅ 588 800°C./Ar/1 hr H₂ + 2% CO₂ 10.318 96.25 0.3% Nb₂O₅ 588 900° C./Ar/1 hr H₂ +2% CO₂ 10.520 98.13 0.3% Nb₂O₅ 588 1000° C./Ar/1 hr H₂ + 2% CO₂ 10.30096.08 0.5% Nb₂O₅ 588 900° C./Ar/1 hr H₂ + 2% CO₂ 10.533 98.25 0.1% TiO₂588 900° C./Ar/1 hr H₂ 10.465 97.62 0.2% TiO₂ 588 900° C./Ar/1 hr H₂10.568 98.58 not added* 588 900° C./Ar/1 hr H₂ + 2% CO₂ 8.0 74.6*comparative example

EXAMPLE 3

The U₃O₈-based powder was prepared in the same way as in Example 1. TheU₃O₈-based powder was mixed with 0.5% niobium oxide (Nb₂O₅) and 0.4%titanium oxide (TiO₂) by weight of the U₃O₈-based powder, respectively.The mixed powder was pre-pressed at a pressure of 98 MPa into slugs,which were then broken up into granules of 425 μm or smaller.

The granules were mixed with zinc stearate at 0.2% by weight of thegranules for lubrication and were pressed into green pellets in a moldunder 588 MPa of pressure. The green pellets were heated to 1700° C. ina reducing gas atmosphere for 4 hours and then cooled-down. The reducinggas was a mixed gas of argon and hydrogen, and the hydrogen gascontained steam of 1.5% by volume of the hydrogen gas.

Table IV shows the densities of UO₂-based pellets fabricated inaccordance with the above procedures. In order to show clearly theeffect of the additives, U₃O₈-based powder containing no additives wasprocessed following the same procedure, and the density of the pellet isalso shown for comparison in Table IV.

TABLE IV additives pressing pressure green density reducing gas sintereddensity sintered density (wt %) (MPa) (g/cm³) atmosph (vol %) (g/cm³) (%TD) 0.4% TiO₂ 588 5.95 Ar + 5% H₂ 10.270 95.80 0.5% Nb₂O₅ 588 5.88 Ar +5% H₂ 10.230 95.43 not added* 588 5.82 Ar + 5% H₂ 7.9 73.7 *comparativeexample

What is claimed is:
 1. A method of producing a UO₂-based fuel pelletfrom an irradiated UO₂-based fuel pellet, the method comprising: heatingan irradiated UO₂-based fuel pellet in an oxidizing gas, to produce aU₃O₈-based product, mixing the U₃O₈-based product with an additivecomprising at least one oxide of an element selected from the groupconsisting of Nb, Ti, V, Al, Mg, Cr, Si and Li, to produce a mixture,pressing the mixture into a green pellet, and sintering the green pelletin a reducing gas atmosphere to reduce the U₃O₈-based product to aUO₂-based product.
 2. The method of claim 1, wherein said heating stepis carried out at a temperature in the range of about 300° C. to about700° C.
 3. The method of claim 1, wherein said mixing step furthercomprises mixing fresh UO₂-based powder with the U₃O₈-based product, theadditive or the mixture.
 4. The method of claim 1, wherein the additiveis mixed with the U₃O₈-based powder in an amount of about 0.02% to 5% byweight of the mixed powder.
 5. The method of claim 1, wherein saidsintering step comprises heating the green pellet to a temperature of atleast 1500° C. for at least 1 hour in a reducing gas atmosphere.
 6. Themethod of claim 1, wherein the reducing gas comprises at least one gasselected from the group consisting of (i) hydrogen, (ii) a mixture ofhydrogen and at least one inert gas, (iii) a mixture of hydrogen andcarbon dioxide, and (iv) a mixture of hydrogen and steam.